1. Field of the Invention
The present invention relates to a video compression technology, and more particularly to a deblocking filter used in a multilayer video encoder/decoder.
2. Description of the Prior Art
With the development of information and communication technologies, multimedia communications are increasing in addition to text and voice communications. The existing text-centered communication systems are insufficient to satisfy consumers' diverse desires, and thus multimedia services that can accommodate diverse forms of information such as text, image, music, and others, are increasing. Since multimedia data is massive, mass storage media and wide bandwidths are required for storing and transmitting the multimedia data. Accordingly, compression coding techniques are required to transmit the multimedia data, which includes text, images and audio data.
The basic principle of data compression is to remove data redundancy. Data can be compressed by removing spatial redundancy such as a repetition of the same color or object in images, temporal redundancy such as little change of adjacent frames in moving image frames or continuous repetition of sounds in audio, and a visual/perceptual redundancy, which considers the human beings' visual and perceptive insensitivity to high frequencies. In a general video coding method, the temporal redundancy is removed by temporal filtering based on motion compensation, and the spatial redundancy is removed by a spatial transform.
In order to transmit multimedia after the data redundancy is removed, transmission media are required, the performances of which differ. Presently used transmission media have diverse transmission speeds. For example, an ultrahigh-speed communication network can transmit several tens of megabits of data per second and a mobile communication network has a transmission speed of 384 kilobits per second. In order to support the transmission media in such a transmission environment and to transmit multimedia with a transmission rate suitable for the transmission environment, a scalable data coding method is most suitable.
This coding method makes it possible to perform a partial decoding of one compressed bit stream at a decoder or pre-decoder end, according to the bit rate, error rate, and system resource conditions. The decoder or pre-decoder can restore a multimedia sequence having a differing picture quality, resolution or frame rate by adopting only a part of the bit stream coded by the scalable coding method.
With respect to such scalable video coding, MPEG-21 (Moving Picture Experts Group-21) PART-13 has already progressed its standardization work. Particularly, much research for implementing scalability in a video coding method based on a multilayer has been done. As an example of such multilayered video coding, a multilayer structure is composed of a base layer, a first enhanced layer and a second enhanced layer, and the respective layers have different resolutions QCIF, CIF and 2CIF, and different frame rates.
FIG. 1 illustrates an example of a scalable video codec using a multilayer structure. In this video codec, the base layer is set to QCIF (Quarter Common Intermediate Format) at 15 Hz (frame rate), the first enhanced layer is set to CIF (Common Intermediate Format) at 30 Hz, and the second enhanced layer is set to SD (Standard Definition) at 60 Hz.
In encoding such a multilayered video frame, the correlation among the layers may be used. For example, a certain area 12 of the video frame of the first enhanced layer is efficiently encoded through prediction from the corresponding area 13 of the video frame of the base layer. In the same manner, an area 11 of the video frame of the second enhanced layer can be efficiently encoded through prediction from the area 12 of the first enhanced layer. If the respective layers of the multilayered video frame have different resolutions, the image of the base layer should be up-sampled before the prediction is performed.
In the present MPEG-21 scalable video coding standard (hereinafter referred to as the SVC standard), research is under way for implementing the multilayered video codec as in the example illustrated in FIG. 1, based on the existing H.264/AVC (Advanced Video Coding).
However, H.264 uses a DCT transform as a spatial transform method, and in a DCT-based codec undesirable blocking artifacts and blocking effect occur as the compression rate is increased. There are two causes of the blocking artifacts.
The first cause is the block-based integer DCT transform. This is because discontinuity occurs at a block boundary due to the quantization of DCT coefficients resulting from the DCT transform. Since H.264 uses a 4×4 size DCT transform, which is a relatively small size, the discontinuity problem may be somewhat reduced, but it cannot be totally eliminated.
The second cause is the motion compensation prediction. A motion-compensated block is generated by copying pixel data interpolated from another position of a different reference frame. Since these sets of data do not accurately coincide with each other, a discontinuity occurs at the edge of the copied block. Also, during the copying process, this discontinuity is transferred to the motion-compensated block.
Recently, several technologies for solving the blocking effect have been developed. In order to reduce the blocking effect, H.263 and MPEG-4 have proposed an overlapped block motion compensation (OBMC) technique. Even though the OBMC is effective at reducing the blocking artifacts, it has the problem that it requires a great amount of computation for the motion prediction, which is performed at the encoder end. Accordingly, H.264 uses a deblocking filter in order to remove the blocking artifacts and to improve the picture quality.
The blocking filter process is performed at the encoder or decoder end before the macroblock is restored and after the inverse transform thereof is performed. In this case, the strength of the deblocking filter can be adjusted to suit various conditions.
FIG. 2 is a flowchart explaining a method for deciding the deblocking filter strength according to the conventional H.264 standard. Here, block q and block p are two blocks that define a block boundary to which the deblocking filter will be applied, and represent the present block and a neighboring block. Five kinds of filter strengths (indicated as Bs=0 to 4) are set according to whether the block p or q is an intra-coded block, whether a target sample is located on a macro-block boundary, whether the block p or q is a coded-block, and others. If Bs=0, it means that the deblocking filter is not applied to the corresponding target pixel.
In other words, according to the conventional method to decide the deblocking filter strength, the filter strength is based on whether the present block, in which the target sample exists, and the neighboring block are intra-coded, inter-coded, or uncoded. The filter strength is also based on whether the target sample exists at the boundary of a 4×4 block or at the boundary of a 16×16 block.
Although the presently proceeding SVC standard adopts a technology that predicts a frame at the present layer using a frame generated at the lower layer, as illustrated in FIG. 1, i.e., an intra BL (Base Layer) mode, it follows the conventional H.264 standard, as illustrated in FIG. 2, when deciding the deblocking filter strength.
However, since the deblocking filter is applied to layers in the video encoder/decoder based on a multilayer, it is unreasonable to strongly apply the deblocking filter again to the frame provided from the lower layer in order to efficiently predict the present layer frame. Nevertheless, in the present SVC standard, the intra BL mode is included as a kind of intra coding and the method for deciding the filter strength, as illustrated in FIG. 2, is applied as is. No consideration is given to whether the present block belongs to the intra BL macroblock when deciding the filter strength.
It is known that the picture quality of the restored video is greatly improved when the filter strength is suitable to the respective conditions and the deblocking filter is applied at a suitable filter strength. Accordingly, it is necessary to research techniques that properly decide the filter strength in consideration of the intra BL mode during the multilayered video encoding/decoding operation.